The long-term objective of this proposal is to elucidate the role of Csk-homologous kinase (CHK) in the breast cancer-related ErbB-2 and Src signaling pathways. The specific aims of this study are to determine 1) the effect of CHK on modification, activation, and function of the ErbB-2 tyrosine kinase receptor cytoplasmic active domain, 2) the effect of CHK on ErbB-2 modification of i) downstream signaling pathway participants; ii) interactions of ErbB-2 with the Src tyrosine kinase; iii) ErbB-2 activation of the ERK-1 and SAPK signaling pathways, and 3) comparative functional analysis of CHK isolated from breast cancer cells/tissues with that isolated from brain and hematopoietic tissues. The studies for specific aims 1 and 2 will utilize recombinant vaccinia virus expression of CHK in 1) NIH3T3 cells which stably express the ErbB-2 cytoplasmic active domain in ligand-dependent or constitutively active forms in the absence of other ErbB-2 pathway participants, and 2) MCF-7 cells, derived from a metastatic breast adenocarcinoma, which endogenously express ErbB-2 and are functional for the ErbB-2 signaling pathway. Specific aim 3 will be accomplished by kinase assay (with known CHK substrates) of CHK-specific immunoprecipitates. Immunoprecipitates of endogenously expressed CHK from the T47D ductal breast carcinoma cell line, CHK-positive breast tumor tissues, brain, and hematopoietic cells will be compared for kinase activity to determine if CHK endogenously expressed in some breast cancers is deficient in kinase activity. The elucidation of the functional role of CHK in breast cancer-specific signal transduction will advance the current understanding of oncogenic signal transduction mechanisms.